This application claims priority of European Patent Application No. 97308743.0, which was filed on Oct. 31, 1997.
1. Field of the Invention
This invention relates to a multiple access protocol for a communication system, and in particular to an asymmetric access protocol for a mobile cellular communication system using time division duplexing (TDD).
2. Description of Related Art
In mobile telecommunication systems, data is passed between a mobile end-user device and a base station in an uplink slot, and vice versa in a downlink slot. Similar half duplex communication protocols are used in other communication systems e.g. a ground station to a satellite. In mobile cellular communication systems, an outstanding problem has been that an asymmetry can occur in the ratio of uplink traffic to downlink traffic. For example, using a mobile end user device to send data (e.g. fax or email) can result in several megabytes to be sent on the uplink, whilst only a few bytes are received on the downlink. Current cellular systems have been designed to support asymmetric traffic ratios which are identical across all cells, which leads to an inefficient use of the frequency spectrum. The guard time between slots is an overhead which is always incurred by the system.
EP766416 A describes a wireless multiple access communication systems which dynamically allocates available frame slots to uplink and downlink. A cancellation signal, corresponding to or otherwise representative of the interfering signal, (from an interfering base station to an interfered-with base station) is used. In the interfered-with base station, the cancellation signal is used to cancel or otherwise offset the mixed interference produced in a received composite signal which includes both the desired mobile station to base station (uplink) signal and the interfering signal. Hence it requires interference cancellation technique (timing adjustment, amplitude adjustment, etc), which results in extra hardware implementation complexity. In addition to uplink and downlink, the base stationxe2x80x94base station link needs to be maintained, hence the involvement of a Base Station Control (BSC), which is higher level over BS. This causes extra signalling traffic and the mobile terminals are especially vulnerable to extra hardware complexity. EP766416 describes a system used purely for cancelling or reducing co-channel interference purposes by the means of slot allocation (between downlink and uplink) in a frequency reuse cell (essentially non-adjacent cells system.
EP 720 321 A1 describes a similar, TDMA only system. The system is used in either non-cellular systems or cellular system with identical asymmetric pattern, that is where the asymmetric ratio of each cell is the same. The systems uses xe2x80x98flexible uplink/downlink allocationxe2x80x99 achieved by system level flexible uplink/downlink allocation, in which the asymmetric ratio is all the same for all cells.
There is thus a need for a flexible and efficient asymmetric multiple access TDD protocol, which increases the amount of traffic that can be supported over a communication channel without adversely affecting the Quality of Service (QoS) experienced by users of the communication system.
According to a first aspect of the invention there is provided a method of time division duplexing in a mobile cellular communication system comprising:
dividing the communication system into a plurality of cells, each cell being served by a base station;
providing a channel in the communication system over which a base station and at least one mobile station may communicate;
using time division duplexing to divide the channel into a plurality of frames, each frame consisting of a plurality of slots;
coordinating the time division duplexing across a plurality of cells; and in which
each base station allocates slots to either uplink communication or downlink communication to optimise throughput in the channel
further comprising
that each frame includes a compulsory uplink communication slot and a compulsory downlink communication slot which may be used by a mobile station in an area where drown out would otherwise occur.
According to a second aspect of the invention there is provided apparatus for time division duplexing in a mobile cellular communication system comprising:
at least one mobile station;
a plurality of base stations, each of which serves a cell of the communications system;
a channel in the communication system over which a base station and at least one mobile station may communicate, the channel being divided, using time division duplexing, into a plurality of frames, each frame consisting of a plurality of slots;
a central controller for coordinating the time division duplexing across a plurality of cells; and in which
each base station allocates slots to either uplink communication or downlink communication to optimise throughput in the channel
further comprising
each frame includes a compulsory uplink slot and a compulsory downlink slot in each frame which may be used by a mobile station in an area where drown out would otherwise occur.
The position of the compulsory slots may be determined by a central controller and the position, within the frame, of the compulsory slots will be the same in each cell.
The invention provides an asymmetric TDD system in which resources are allocated on a cell by cell basis, thereby achieving a more flexible and efficient system. All data packets transmitted in a slot can be arranged into consecutive slots; that is uplink slots and downlink slots can be arranged consecutively, thereby obviating the need for guard time between the same type of slot. Since the invention allows an asymmetric traffic ratio (uplink:downlink) for each cell the channel efficiency is improved. Since each base station may allocate uplink and downlink slots according to the type of traffic experienced by a cell, the traffic ratio may be adjusted dynamically.
The slots may be arranged into frames, with the asymmetric traffic ratio being repeated on a frame by frame basis. The base station may broadcast the traffic ratio (i.e. the uplink and downlink allocation) on a general information channel BCCH, so that each mobile station entering a cell is provided with the traffic ratio.
The central coordinating controller may also organise the length of a frame, and govern the timing of frames from cell to cell. Each frame may include a compulsory uplink slot and a compulsory downlink slot, which may for example take the first two slots of each frame. The compulsory slots may be the same for each cell, leading to easy handover between cells, for a mobile station travelling between cells. The frame rate and compulsory slot allocation may be chosen in such a way that a mobile station, restricted to the compulsory slots only, will not suffer from a reduced QoS, particularly if running a delay sensitive application.